The adhesive tape of almost every heat-activatable adhesive tape now on the market has a rather narrow melting point range. Such a tape is called a "hot-melt" adhesive tape, because its adhesive layer is melted at the bonding temperature. At temperatures within 10.degree. C. below the bonding temperature, the adhesive may not even be tacky. Because the adhesive has virtually no strength at the bonding temperature, a clamp may be used until a strong bond develops, either by cooling or by curing. If the bond is later heated to a temperature at or above the bonding temperature, the bond fails unless the adhesive has become cured, e.g., by crosslinking. If the adhesive becomes cured when heated, the tape tends to have a limited shelf life, especially if stored in ordinary warehouses which may become quite hot during the day.
Pressure-sensitive adhesive tapes provide immediate bonding strength ("wet-grab") and the bonds tend to retain good strength at moderately elevated temperatures, but may weaken to the point of failure at higher temperatures. An adhesive which is virtually nontacky at 20.degree. C. or other ordinary room temperatures may become pressure-sensitive and provide immediate bonding strength at elevated temperatures. Such an adhesive may be called a "hot-tackifying" adhesive. When later subjected to temperatures at or somewhat above the bonding temperature, adequate bond strength is retained.
Hot-tackifying adhesives are particularly useful in tape constructions disclosed in U.S. Pat. No. 3,718,495 (Tomita) and U.S. Pat. No. 3,475,213 (Stow). The Tomita tape has a heat-shrinkable backing which when heated shrinks tightly around an object onto which it has been wrapped. Upon prolonged exposure to elevated temperatures, an ordinary pressure-sensitive adhesive can become so soft that the outermost wrap may become loose. Hot-tackifying adhesives are more resistant to softening at elevated temperatures. The tape of the Stow patent has an electrically-conductive backing and an adhesive layer containing electrically-conductive metal particles which preferably are flattened and may comprise from 0.1 to 40 volume percent of the adhesive layer. When applied to electrical conductors such as are on one face of a printed circuit board, the particles conduct electricity between the conductors and the backing. If the adhesive were to become soft when heated, as by electricity flowing through the metal particles, the particles would be able to move about, gradually resulting in increased electrical resistance unless the assembly has a permanent clamp, but this can be awkward and expensive.
A preferred class of "hot-tackifying" adhesives is provided by copolymers of alkyl acrylates and/or methacrylates and copolymerizable monomers having strongly polar groups such as acylic acid, itaconic acid, acrylonitrile, and acrylamide. When those hot-tackifying adhesive tapes are heated and applied to inorganic substrates (e.g., glass, ceramic, metal), the bonds tend to fail upon prolonged exposure to high humidity at elevated temperatures.